Not Applicable.
The present invention relates to power semiconductors and, in particular, to a microelectromechanical system (MEMS) device providing on-board current and voltage sensing capabilities for power semiconductors.
Many circuit applications in use today employ semiconductor devices to control large currents and voltages. The measurements of these voltages and currents are crucial to determine, for example, average current flowing, RMS current, peak current, average voltage, RMS voltage, or peak voltage. The measured value may be used for device protection, closed loop control, or many other purposes. Because the voltage or current may typically not be referenced to the same circuit point as the rest of the electronics, isolation of the measurement is usually necessary. Furthermore, it is desirable to minimize the cost associated with the measurements.
A conventional current sensor 25 used with a power semiconductor 11 is illustrated in FIG. 1, wherein a plurality of transistor elements 19 are fabricated on a single chip and are connected in parallel to a source 23 to control a load 17. An external current sensor 25, which may take the form of shunt resistors and associated circuitry, current transformers and associated circuitry, Hall Effect sensors and associated circuitry, and the like, is connected in series with the transistor elements 19. The transistor elements 19 are connected in parallel so as to limit the amount of current that is required to flow through each transistor element. Disadvantageously, the sensor 25 requires external connection to the circuit and subsequent calibrated by the end user for reliable operation.
Likewise, a conventional voltage sensor used with a power semiconductor 11 is illustrated in FIG. 2. The voltage across the transistor elements 19 is sensed and measured by sensor 27.
Electrical isolators are used to provide electrical isolation between circuit elements for the purposes of voltage level shifting, electrical noise reduction, and high voltage and current protection.
Circuit elements may be considered electrically isolated if there is no path in which a direct current (DC) can flow between them. Isolation of this kind can be obtained by capacitive or inductive coupling. In capacitive coupling, an electrical input signal is applied to one plate of a capacitor to transmit an electrostatic signal across an insulating dielectric to a second plate at which at output signal is developed. In inductive coupling, an electrical input signal is applied to a first coil to transmit an electromagnetic field across an insulating gap to a second coil, which generates the isolated output signal. Both such isolators essentially block steady state or DC electrical signals.
Such isolators, although simple, block the communication of signals that have significant low frequency components. Further, these isolators can introduce significant frequency dependent attenuation and phase distortion in the transmitted signal. These features make such isolators unsuitable for many types of signals including many types of high-speed digital communications.
In addition, it is sometimes desirable to provide high voltage ( greater than 2 kV) isolation between two different portions of a system, while maintaining a communication path between these two portions. This is often true in industrial control applications where it is desirable to isolate the sensor/actuator portions from the control portions of the overall system.
What is therefore needed is an isolated on-board current and/or voltage sensor that may be pre-calibrated during manufacturing to simplify use to the end user and that provides an isolated output.
In accordance with one aspect of the invention, an on-board microelectromechanical system (MEMS) electrical device is provided for sensing an electrical property of a power transistor having a collector, a base, and an emitter. Henceforth these terms will be used to indicate generally a power input, a control input, and a power output and thus indicate, generically, any power semiconductor and embrace the terms drain, gate, and source, and other terms for such terminals of these devices. It should be further appreciated that the present invention is intended to be applicable to sensing electrical properties of diodes, which traditionally only have two terminals (anode and cathode).
In one preferred form, the device comprises 1) a substrate; 2) an element supported by the substrate and movable between a first and second position; 3) an actuator attached to the element to receive an input electrical signal from the transistor that exerts a force dependent on the input electrical signal urging the element toward the second position; 4) a bias structure attached to the element to exert a predetermined opposite force on the element urging the element towards the first position; and 5) a sensor attached to the second portion of the element to provide an output electrical signal indicating movement of the element between the first position and the second position.
This and other aspects of the invention are not intended to define the entire scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, and not limitation, a preferred embodiment of the invention. Such embodiment does define the scope of the invention and reference must therefore be made to the claims for this purpose.